In the course of a number of diseases such as renal, ureteric, salivary duct and gall stone disease, patients may develop hard deposits, known as ‘stones’. One method of treating these stones is Extracorporeal Shockwave Lithotripsy (SWL or ESWL). This technique uses acoustic shockwaves generated outside of the body to fragment the stones to a size sufficiently small that they can be passed normally through the body, or so that they can be dissolved using drugs. Several mechanisms are likely contributors to stone fragmentation during SWL. These can be broadly divided into direct stress and cavitation. Direct stress refers to the impact of the shock wave on the stone and the subsequent evolution of stress inside the stone (e.g. spallation, and the formation of shear waves by the incident shock squeezing the stone and the interaction of internal waves in the stone with the surface). Cavitation refers to the growth and collapse of bubbles, for example in the urine surrounding the stone, as a result of the shock from the SWL device. SWL is presently used for the non-invasive treatment of 90% of all kidney stones. However, this method of treatment has a number of associated limitations.
The success of an SWL treatment is currently assessed using ultrasound or X-ray fluoroscopy interrogation. However, it is extremely difficult for an operator to determine whether a stone has been fragmented based on the images produced using such methods, partly because the fragments of a treated stone may stick together following a treatment. Furthermore, the assessment cannot be carried out in real time, for example during the treatment. As a result, SWL treatments tend to follow a ‘one size fits all’ regimen of, typically, 3000 shocks. As each shock will result in some collateral damage to the surrounding soft tissues, this situation will tend to result in unnecessary damage in patients who might require fewer shocks. In some situations where the treatment has been ineffective, the collateral damage may have been sustained with no benefit to the patient, who will then require repeat or alternative treatment; retreatment rates are currently at 30-50%. Finally, the use of active X-ray fluoroscopy radiation or acoustic waves also exposes the patient to further radiation, in addition to that required for the SWL treatment.
These problems arise largely because it is difficult to align the focus of the lithotripter acoustic beam directly towards the stone, and the performance of the treatment system therefore depends on the size and location of the stone. The alignment, in the absence of a real-time monitoring method, cannot be checked during the treatment unless the treatment is stopped for intermittent check using X-rays or ultrasound. If the stones moves out of the focus during treatment, then in the absence of a real-time monitoring method this would not be detected unless the treatment were stopped.
It is an object of the present invention to mitigate the problems outlined above.